Methods of Application

The type of rust preventive and the quantity, size, complexity, and surface finish of the articles to be coated will determine which method should be used. The equipment and methods of application are similar to those used in painting.

Petrolatum compounds can be applied either hot or cold. Generally, cold application is restricted to parts that are either too large and bulky for practical tank immersion or require only localized protection, such as the ways for a lathe bed. When these compounds are applied cold, their consistency requires the use of brushing or wiping. When brushing is used, the bristles of the brush should be stiff enough to permit brush-out of the material, but not so stiff as to leave deep brush marks in the preservative. With these materials, it is often advantageous to build up coatings to the desired final thickness by the successive application of thin layers.

When petrolatum rust-preventive compounds are applied hot, dipping is the most practical method. Parts that are too large to be dipped can be coated by brushing, wiping, or spraying. Small parts to be dipped are placed in baskets, whereas larger parts are individually dipped. Dipping can be manual, when the volume of parts is low; conveyors can be used when parts are produced in large quantities.

Most dipping tanks for petrolatum compounds are heated by steam coils, hot-water jackets, electrical immersion heaters, or plate coils using either hot water or steam. Although precise temperature control is not usually required, the compound in the dipping tank must not become overheated. Overheating can cause decomposition of the preservative, and the resulting products may act as corrosion agents. Some petrolatum products have been applied at elevated temperatures electrostatically, which can afford better control of the amount of coating being applied.

Oil compounds can be applied electrostatically or by dipping, spraying, flowing, brushing, or wiping. The thickness of the coating depends on the viscosity, fluid characteristics (whether the fluid is Newtonian or non-Newtonian), and surface tension of the oil compound. Oils of high viscosity usually are heated before being applied. Parts or assemblies coated by immersion in oil preservatives should be turned or agitated to permit all trapped air to escape. When the material is applied at room temperature, the parts will require immersion only long enough to ensure complete coverage.

For spray or electrostatic application, oil-based rust-preventive compounds usually do not require dilution and are applied as received. Moderate air pressure or an airless spray is used to avoid misting and overspraying of the material. A wetting spray is usually sufficient. It is more difficult to control coating weight and uniformity by spray application than by dipping or by electrostatic application in coil or sheet stock. To ensure an adequate coat, the material should be applied according to the specifications supplied by the end user. Oil-based compounds used on coil or sheet stock are applied by using the roll-coater spray and electrostatic methods, which allow some control of the coating weight. After a spray application, squeegee rolls are used. Changing the pressure on the squeegee can help control the amount of film on the surface.

Emulsion compounds are oil-in-water emulsions that contain from 8 to 12% solids. These compounds, which are available as concentrates, are diluted with water at ratios of 4 to 10 parts water to 1 part concentrate, as specified in MIL-C-40084.

These compounds, which are widely used for small parts, are applied by dipping or spraying. The effectiveness of the application can be increased by heating either the parts or the compound. For example, the unheated compound can be applied in the third stage of a power washer in which the parts retain residual heat produced during the first two stages. If dipping is used, then the compound can be heated to a gentle boil before immersion of the unheated parts.

Oil-in-water emulsions are fire resistant during application. However, after the water has been removed by drying, the residual film has flammability characteristics that are comparable to those of petroleum oils or waxes.

Solvent-cutback compounds are essentially waxes modified with high-melting-point polar additives, such as soaps. Compounds that use organic solvents as the diluent are applied at room temperature. Small parts usually are batch coated in wire baskets, which are immersed, withdrawn, and centrifuged to remove excess material. Commercial equipment is available for this method of application. The parts are then removed from the basket and are either spread out to dry or conveyed through an air-drying stage, which may be mildly heated.

Because some solvent-cutback compounds develop dry, hard films, parts may become bonded together by the film. Separating these parts often results in tears in the coating. Large parts can be spray or dip coated, but all solutions are not necessarily applied equally well by both methods. Parts that are too large for dipping can be coated by spraying or brushing.

When this type of product includes polar materials, it can be used on coil and sheet stock to protect edges and other exposed portions during shipping and handling. Spray, brush, or paint-roller-type applicators can be used. High-flashpoint vanishing oils (solvent cutback) have also been applied electrostatically. However, extreme caution should be taken to ensure against arcing. The equipment manufacturer should be consulted prior to testing or use.

Water-displacing compounds are most effective when they are applied by dipping, although spraying also can be used. These materials actually remove films or droplets of water from metal surfaces by preferential wetting, that is, the attraction of the rust-preventive compound to the metal surface is greater than that of water and, thus, the preservative displaces the water. Complete immersion of the part makes the coverage more positive.

A suitable dip tank for automatic water removal from a water-displacing preservative system is shown in Fig. 1. Because the specific gravity of the preservative is less than that of water, the height of the column of water is somewhat less than the height of the column of preservative. Parts that are too large to coat by immersion and too impractical to spray can be coated by wiping. Wiping is also used when only certain areas of parts need to be coated. Coiled and sheet stock can be electrostatically coated.

Fingerprint removers and neutralizers can be applied by spraying, dipping, or brushing, the latter of which is usually used only when a section of a part needs to be coated. These materials are applied at room temperature. Sufficient fluid should be present in the system to prevent gross contamination or preferential depletion of the active ingredients that dissolve or neutralize the corrosive materials.